As the problem of emission of carbon dioxide into the atmosphere and the energy problem became obvious, polylactic acid, which is a biodegradable polymer, has been drawing attention in view of its sustainability and as a life cycle assessment (LCA)-oriented product. At present, polylactic acid is mainly produced as L-lactic acid polymers, and highly optically pure L-lactic acid is now being mass produced as the raw material for the polymers.
Further, as polylactic acids having high heat resistance, stereocomplex polylactic acids (Sc-PLAs) are drawing attention. Since Sc-PLAs require as raw materials not only L-lactic acid but also D-lactic acid, which is the optical isomer of L-lactic acid, methods of producing highly optically pure D-lactic acid are now being developed.
Since, as mentioned above, high optical purity is required for lactic acid as a raw material for the polymers, the method of producing the lactic acid is currently limited to lactic acid fermentation by microorganism culture. However, in such a case, the microorganism needs to be cultured while the pH optimum for production of lactic acid by the microorganism (hereinafter referred to as the lactic acid fermentation optimum pH) is maintained. For example, in cases where a lactic acid-producing yeast having a lactic acid fermentation optimum pH of about 4.5 is cultured to produce lactic acid, the pH of the culture liquid needs to be kept at the lactic acid fermentation optimum pH by alkali neutralization to maintain the lactic acid productivity of the lactic acid-producing yeast (see JP 2009-171879 A). If the culture liquid of the lactic acid-producing yeast is neutralized with an alkaline substance for this purpose, by-products such as gypsum may be produced in the subsequent step of separation/purification of lactic acid, depending on the amount of alkaline substance added. This makes the step of separation/purification of lactic acid laborious, resulting in high cost of production of lactic acid. Further, although there was a case where a low-pH-resistant lactic acid-producing yeast was used to produce lactic acid at pH 3, which is a pH lower than the lactic acid fermentation optimum pH of the lactic acid-producing yeast, the lactic acid productivity obtained at a pH lower than the lactic acid fermentation optimum pH is lower than the lactic acid productivity obtained at the lactic acid fermentation optimum pH, which is problematic (see Japanese Translated PCT Patent Application Laid-open No. 2009-517045).
It could therefore be helpful to provide a lactic acid-producing yeast mutant which is resistant to the low-pH state of the culture liquid caused during lactic acid production by culture of a lactic acid-producing yeast, which lactic acid-producing yeast mutant has, in a low-pH state, a lactic acid-producing ability equivalent to or higher than the lactic acid-producing ability at the lactic acid fermentation optimum pH, and a method of producing lactic acid using the mutant.